Nuclear Physics B Proceed- ings Nuclear Physics B Proceedings Supplement 00 (2018) 1–6 Supple- ment Direct probes of neutrino mass R. G. Hamish Robertson Center for Experimental Nuclear Physics and Astrophysics and Dept. of Physics, University of Washington, Seattle WA 98195, USA Abstract The discovery of neutrino oscillations has shown that neutrinos, in contradiction to a prediction of the minimal standard model, have mass. Oscillations do not yield a value for the mass, but do set a lower limit of 0.02 eV on the average of the 3 known eigenmasses. Moreover, they make it possible to determine or limit all 3 masses from measurements of electron-flavor neutrinos in beta decay. The present upper limit from such measurements is 2 eV. We review the status of laboratory work toward closing the remaining window between 2 and 0.02 eV, and measuring the mass. Keywords: Neutrino mass, beta decay, tritium, electron spectroscopy 1. Introduction conservation in the weak interaction in 1956 [6] and the measurement of the helicity of the neutrino in 1958 [7] The mass of the neutrino has been a topic of specula- made plausible the idea that the neutrino was massless, tion and research since the theory of beta decay was for- and massless neutrinos were subsequently built into the mulated by Fermi [1]. Neutrino mass affects the shape standard model. A decade-long hiatus in searches for of the beta spectrum, and even with the limited data neutrino mass followed. A second kind of neutrino, available at the time, Fermi was able to conclude that the muon neutrino, was discovered in 1962 [8], and the mass of the neutrino must be “either zero, or in any a third, the tau neutrino, was found to exist in 1975 case very small, in comparison to the mass of the elec- [9]. Interest in direct searches for neutrino mass re- tron” [2]. The discovery by Alvarez and Cornog [3] in vived once it was realized that neutrinos were not re- 1939 that tritium was radioactive and had a small Q- sponsible for parity violation because the neutral cur- value was important because the effect of neutrino mass rent [10] also violated parity [11]. A flurry of excite- is relatively greater in that case. Moreover, tritium has ment pervaded the neutrino-mass community when in a simple atomic structure, a uniquely valuable property 1981 Lyubimov et al. [12] reported that the electron an- as the sensitivity of neutrino mass experiments has ad- tineutrino had a mass of 30 eV, a value that closes the arXiv:1502.00144v1 [nucl-ex] 31 Jan 2015 vanced over the years. Seventy-five years later, tritium universe gravitationally and would explain its observed remains the isotope of choice in the continuing quest to flatness. Unfortunately this result turned out to be in- measure the mass of the neutrino. correct, as was shown by a group at Los Alamos [13]. The first neutrino mass determinations from the shape The Los Alamos experiment used a source of gaseous of the tritium spectrum were carried out with propor- T2, because Bergkvist had observed in 1972 [14] that tional counters in 1948 by Hanna and Pontecorvo [4] at experiments had reached a sensitivity where it becomes Chalk River and by Curran et al. [5] in Glasgow. Hanna essential to consider molecular and atomic excitations and Pontecorvo were able to set a limit of 500 eV on in interpreting the spectrum. It was almost certainly the the neutrino mass, Curran et al. about 1 keV. Experi- complexities of the tritiated valine molecule that con- mental work continued until the discovery of parity non- tributed to the erroneous result of Lyubimov et al. 1 R. G. H. Robertson / Nuclear Physics B Proceedings Supplement 00 (2018) 1–6 2 The discovery of neutrino oscillations in atmospheric, minimal standard model. The standard model is known solar, and reactor neutrinos [15, 16, 17] brought a pro- to be incomplete, lacking things like gravity and dark found change. Neutrinos were shown to have mass, and matter, but until neutrinos were found to have mass, it a lower limit on the average mass of the three eigen- had never produced a disagreement with data. Under- states (0.02 eV) has been established. All three masses standing the origin and patterns of neutrino mass is thus are linked by small differences, which means that direct of great interest, because new physics at a very high searches can focus on the experimentally most accessi- mass scale may be responsible. Neutrinos also play a ble neutrino (the electron antineutrino) and will be able significant role in the formation of the universe. They to determine the 3 masses from a single measurement are only a small fraction of the dark matter, but because (with a two-fold ambiguity from the presently unknown they cool from relativistic to non-relativistic at a recent hierarchy). The quantity measured in a beta decay ex- epoch in the last few billion years, they have influenced periment is a weighted sum of eigenmasses: large-scale structure. Quantifying that influence is de- X sirable, and a laboratory measurement would free cos- m2 = jU j2 m2 ; (1) β ei νi mologists from the need to include neutrino mass in fits i=1;3 to extract other parameters that can only be obtained where the mνi are neutrino eigenmasses and U is from astronomical observation. the Maki-Nakagawa-Sakata-Pontecorvo mixing matrix In this review the status of experiments now in [18]. The absolute lower bound, which neutrino oscil- progress is considered. There are two experiments on ≥ 2 lation measurements provide, is mβ 0:01 eV/c for tritium. The KATRIN experiment in Karlsruhe is in the ≥ 2 the normal hierarchy, and mβ 0:05 eV/c for the in- final stages of construction. A new scheme, Project 8, verted hierarchy. When the mass is larger than roughly is under exploratory development in Seattle. There are 2 ' ' 0:1 eV/c , the ‘quasi-degenerate’ regime, mβ mν1 also ideas about using 187Re or 163Ho embedded in mi- ' mν2 mν3. Figure 1 shows a steady ‘Moore’s Law’ crocalorimeters but the Re decay is hindered, calling for decrease in the upper limit on neutrino mass from ex- large amounts of this costly element, and the Ho spec- periments spanning more than 60 years. trum has a complex shape [21]. 1000 2. The KATRIN experiment 100 HDM Ω = 1 The KArlsruhe TRItium Neutrino experiment (KA- TRIN) couples a gaseous T2 source to a large spectrom- 10 eter, a ‘MAC-E’ filter based on Magnetic Adiabatic Col- limation with Electrostatic retardation. The experiment 1 is located on the north campus of the Karlsruhe Institute mass limit, eV β of Technology, contiguous with the prototype tritium- m 0.1 IH lower limit handling facility developed for the ITER controlled fu- sion experiment now under construction in France. The NH lower limit 0.01 apparatus concept is shown in Fig. 2. The basic op- 1950 1960 1970 1980 1990 2000 2010 2020 eration of the experiment begins with gaseous molec- ular tritium recirculated through the WGTS at a tem- Year perature of 27 K maintained by a two-phase neon cool- ing loop. A solenoidal field guides electrons from the Figure 1. Upper limits on neutrino mass obtained from tritium beta decay experiments vs. year. The point with error bars is the non-zero source through several stages of pumping and into a pre- value reported by Lyubimov et al. [12]. Also indicated by HDM is spectrometer that rejects all but the last 100 eV or so of the mass that would close the universe in the absence of other contri- the spectrum. Electrons that surmount the prespectrom- butions, and the lower bounds set by oscillations for the inverted and normal hierarchies. eter potential enter the main spectrometer, which has an energy resolution of 0.93 eV base width. If they also The current upper limit on neutrino mass comes from surmount the main spectrometer potential, the electrons two tritium experiments, one at Mainz [19], and the are transmitted to a multipixel Si detector for counting. other at Troitsk [20]. The experiments lead to a con- A spectrum can be built point by point, by stepping ei- cordant limit of about 2 eV [18]. ther the main spectrometer potential or the potential of Neutrino mass is an important question in physics for the WGTS. A comprehensive description of the concept two reasons. It is the first definitive contradiction to the of KATRIN can be found in Ref. [22]. 2 KATRIN monthly report: Executive Summary November 2014 INTECH GmbH, Geitnerweg 21, 12209 Berlin. KIT Project Management KIT contractor for support in ProjectR. G. management H. Robertson / Nuclear Physics B Proceedings Supplement 00 (2018) 1–6 3 KIT – University of the State of Baden-Wuerttemberg and FigureNational 2. OverviewResearch Center of theof the KATRIN Helmholtz apparatus.Association Functional units (from left): Rear system with calibration devices (RS); differentialwww.kit.edu pumping section (DPS1R); windowless gaseous tritium source (WGTS); differential pumping section (DPS1F); differential pumping section with magnetic chicane (DPS2F); cryogenic pumping section with argon frost at 3K (CPS); prespectrometer; main spectrometer; detector. On site and complete are the prespectrometer, main upper limit of 0.2 eV at 90% CL after 5 calendar years, spectrometer, and detector. With an electron gun source, or a discovery at 5σ of 0.35 eV. the main spectrometer and detector have been undergo- ing commissioning tests. The cryogenics for the WGTS have been built and tested: the system performs very 3. Project 8 well, with temperatures controlled to 4 mK at 27 K As Bergkvist pointed out in 1972, the final-state spec- [23].